Satellite transmissions for commercial television are broadcast from satellites in geosynchronous orbits to satellite antenna systems designed to receive signals in the 5-30 GHz range. For example, Direct Broadcast Satellite (DBS) transmissions are delivered at the 11-to-15 GHz frequency range, known as the Ku-band, to dish-shaped antennas. While antennas for receiving satellite television signals can measure up to sixty inches or more in diameter, current satellite dish antenna designs are generally less than thirty-six inches in diameter. The most common DBS satellite television dish antennas are only about eighteen inches in diameter (e.g., DirecTV®, Dish Network (Echostar), Bell Express View).
The antenna dish concentrates and reflects satellite microwave signals that strike the antenna dish back to a focal point that is in front of the antenna dish. A feed horn is positioned on a support arm at the focal point to route the microwave signals to a signal converter that converts the microwave signals into electrical signals. These electrical signals are then provided to a satellite receiver that translates the electrical signals into a television picture and sound.
A long-standing and still unresolved problem with satellite reception is signal loss and signal fade caused by weather conditions, such as rain, ice, dew, wind or snow (hereinafter referred to as “rain fade”). As any viewer of a satellite television system can confirm, the presence of moisture interferes with and often totally blocks good satellite signal reception. Many attempts have been suggested over the years to improve satellite reception in the face of various weather conditions.
For larger antennas, the use of domes, radomes or other types of covers to protect the antenna has been a common approach to addressing this problem. Most domes or radomes for satellite dishes are hemispherical shells made of fiberglass or other similar materials that will interfere with the transmission of microwaves as little as possible. U.S. Pat. No. 4,804,972 describes a clamshell configuration for a hard radome. U.S. Pat. No. 4,946,736 describes laminated panels for use in a large radome made of porous expanded polytetraflouroethylene (Teflon®). U.S. Pat. Nos. 3,388,401, 4,918,459 and 5,451,972 describe examples of flexible fabric covers that cover just the dish for a larger antenna. U.S. Pat. No. 5,528,253 describes a flexible fabric cover that also extends over the support arm and feed horn.
In addition to the antenna, a plastic cover or protective shield has long been used for protecting the entry to the feed horn itself as shown, for example, in U.S. Pat. No. 3,781,898. There is a critical relationship between wavelength of the microwaves and the design of the feed horn cover that impacts the ability of the signal converter to convert the microwaves received at the feed horn into electrical signals. This electromagnetic relationship is described in U.S. Pat. No. 5,675,348, that teaches even the thickness of the feed horn cover can affect the signal strength provided to the signal converter. As a result of the desire to protect this critical relationship, approaches for protecting or covering the feed horn have focused on providing brims or hoods that serve as rain deflectors for the feed horn as shown, for example, in U.S. Pat. Nos. 4,282,530 and 6,072,440. Most of the attempts to address the problems of rain fade, however, have focused on covering or shielding the satellite dish or to the entire satellite system. Although these solutions can address the problem to a certain extent, none of these attempts have prevented the intermittent reception, and sometimes total loss of satellite television signal during adverse weather conditions.
While a radome or cover over the antenna dish necessarily reduces the overall satellite signal strength the antenna receives, a reduction in overall signal strength in exchange for a more consistent signal during weather conditions can be acceptable for larger antennas where the initial signal strength is sufficient to tolerate such a reduction. This is due in part to the squared relationship between the diameter of a dish antenna and the surface area of that dish antenna that serves to reflect and focus the incoming satellite signals back into the feed horn that is connected to the actual circuitry of the satellite receiver. An antenna dish with ½ the diameter will have only about ¼ of the surface area to collect and reflect the signal back into the feed horn.
Even though most newer dish designs for DBS satellite television systems have diameters that are only one-third the size of larger, older DBS antenna systems (and therefore collect only about one-ninth of the satellite signal energy), various domes, radomes and covers have been proposed to address the problems of weather and rain fade for these smaller DBS satellite antennas. U.S. Pat. Nos. 5,729,241 and 6,714,167 describe flexible covers for just the dish antenna of a DBS satellite television system. U.S. Pat. No. 5,815,125 describes a flexible cover for a DBS antenna system that covers both the antenna and the feed horn. U.S. Pat. No. 5,877,730 describes a hard brim extending out from the dish of a DBS satellite antenna. U.S. Pat. No. 6,538,612 describes a dome for a DBS satellite antenna for a mobile home that includes an automated satellite locator system to orient the dish antenna under the dome. U.S. Pat. No. 6,191,753 describes a hard cone-shaped cover for both dish and feed horn of a DBS satellite antenna. Not surprisingly, none of these arrangements has provided a satisfactory or viable solution to the problem of satellite rain fade.
Part of the reason for the failure of domes, radomes and covers to solve the problem of rain fade can be traced to the absorptive and reflective properties of water that builds up on the surface of the dome, radome or cover. These effects were first analyzed in experiments conducted more than thirty years ago. Anderson, “Measurements of 20-Ghz Transmission Through a Radome in Rain,” IEEE Transactions on Antennas and Propagation, Vol. AP-23, No. 5, September 1975.
In recognition of these effects, almost every type of commercially available “water repellant” or “waterproof” coating has been suggested for use on domes, radomes, antennas and antenna covers as a way of shedding or repelling water in order to minimize these effects. For example, U.S. Pat. Nos. 5,357,726 and 5,368,924 describe improved waterproof fabrics for use as antenna covers. U.S. Pat. No. 6,292,155 even describes the use of hydrophobic tear-off protection sheets for a DBS satellite dish. While these kinds of water repellant or hydrophobic coatings or materials can provide some degree of improvement, none have been able to solve the problems of satellite signal rain fade, particularly for the newer, smaller DBS antenna dishes.
In the last few years, a new class of water repellant or hydrophobic materials known as superhydrophobic materials have been developed. Measurements for how water repellant a material will be utilize a measurement of the static contact angle between a drop of water and the surface of the material. For materials with a static contact angle of less than 90° generally form a sheet or film in response to heavy moisture. Most hydrophobic materials have a static contact angle of between about 90° and 120°. Heavy rain tends to form rivulets on the surface of a hydrophobic material allowing the water to run off as large drops or streams. Superhydrophobic materials have static contact angles greater than about 1200 and often greater than 140° to 150°. Heavy rain tends to roll off the surface of a superhydrophobic material as very small drops rather than slide off.
Examples of these kinds of superhydrophobic or extremely hydrophobic coatings are described in U.S. Pat. No. 6,663,941 describing flourothane superhydrophobic materials and U.S. Pat. No. 6,683,126 describing a nanoparticle powder dispersed in a binder that creates a superhydrophobic effect. Other examples of superhydrophobic materials include: Vellox™ LC-410 treated silica paint, Nanosil nanoscale surface texturing, treated isotactic polypropylene, a silica polystyrene film treated with flouroalkysilane, a poly(tetraflouroethylene) (PTFE) film where the density of crystals in the film are decreased by axial extension of the film, and a low percentage titanium oxide film.
Some of the descriptions of these various superhydrophobic materials suggest that they can be used in coating satellite dish antennas. Unfortunately, it has been discovered that coating the satellite antenna dish with a superhydrophobic material does not have a significant impact on the problem of rain fade, particularly for the smaller DBS satellite television antennas. Thus, there is a continuing, unmet need for a satellite antenna system that can create more reliable satellite television reception in conditions where moisture would otherwise interfere with satellite television reception.